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transition-interpolator-class-rfc.md

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Viewport Transition Interpolator Class RFC

Motivation

Users may want to make slight modifications to the behavior of a built-in interpolator without writing too much code.

Example Use case

viewportLinearInterpolator transitions all of latitude, longitude, zoom, pitch and bearing. During viewport transition, an application only wants to interpolate a subset of the props (e.g. pitch and bearing).

Problems in the Current Solution

The solution introduced in PR1111 added a new prop transitionProps to the viewport controller. Unfortunately, this introduced new issues. FlyToInterpolator and viewportLinearInterpolator require different transitionProps. FlyToInterpolator requires width and height to be included to calculate the transition correctly. viewportLinearInterpolator by default leaves out width and height to avoid expensive updates to the canvas size. Current multi-viewport implementation also makes width and height unavailable to the Viewport class as they are calculated outside of viewport. Therefore, significant knowledge regarding the specific interpolator classes is required for user to provide valid values to viewport controller and providing a good default value for transitionProps also becomes a challenge. Failing to conform to the hidden requirement of transitionProps will cause the application to break (FlyToInterpolator checks the extracted viewport, which is set by transitionProps, and throws an error if any required field is missing).

Another concern is that transitionProps is specific to viewportLinearInterpolator. We may encounter other use cases in the future. Adding a new prop every time will make the API more cluttered, harder to debug, and affect existing applications and interpolators in unintended ways.

Proposal

There are 3 key steps in TransitionManager that define the transition behavior:

  • compare viewports - determines whether a new transition should be triggered.
  • extract viewport - saves start and end viewports for use in the interpolation.
  • interpolate viewport - given the start and end viewports and a time factor between [0, 1], outputs the viewport props in transition.

This RFC proposes:

  1. Add a ViewportTransitionInterpolator class that has the following methods:

    • areViewportsEqual(startViewport, endViewport)
    • extractViewportFromProps(props)
    • interpolateViewports(startViewport, endViewport, t)

  2. Change the current controller prop transitionInterpolator from a function to an instance of ViewportTransitionInterpolator. Remove the transitionProps prop.

  3. Replace the experimental exports viewportLinearInterpolator and FlyToInterpolator with classes ViewportLinearInterpolator and FlyToInterpolator that extend ViewportTransitionInterpolator. To use them, the user would supply:

<MapController transitionInterpolator={new FlyToInterpolator()} />

An interpolator class may also offer further customization:

<MapController transitionInterpolator={new ViewportLinearInterpolator(TRANSITION_PROPS)} />

Benefits

  1. The proposed ViewportTransitionInterpolator class abstracts away two existing props that have dependency on each other. It will also help keep the API stable as we add new customization features in the future.

  2. Once this proposal is implemented, TransitionManager should be completely viewport agnostic. In time, we will be able to provide new ViewportTransitionInterpolator classes to handle transition of first-person or non-geospatial viewports.